Color television systems have been developed using several different broadcast and signal processing formats to achieve the successful transmission and reception of color television programming. While substantial differences between systems exist, they all must satisfy the basic objective of combining the picture or luminance information, the color or chrominance information, and sound information together with appropriate display scan synchronizing signals to form an information signal which may be modulated upon a carrier for transmission. At the receiver, the opposite processes must take place in which the several components of the information signal are separated and appropriately processed. In most television broadcast formats such as the NTSC system used within the United States of America and the PAL system used in many European countries, the signal components corresponding to luminance, chrominance and sound are distinguished from each other and separated for individual processing largely on the basis of signal frequencies.
In the NTSC system, for example, the available broadcast bandwidth is maintained at 6 megahertz. To conserve channel bandwidth and to transmit up to 4.1 megahertz of video signal, a vestigial sideband format in which the carrier is off center within the 6 megahertz channel bandwidth is used. The chrominance information is phase and amplitude modulated and is modulated upon a chrominance subcarrier separated from the picture carrier by approximately 3.58 megahertz. The chrominance modulation system is known as "suppressed carrier" modulation so named because the chrominance information sidebands are transmitted without the chrominance subcarrier itself. To facilitate the regeneration of the chrominance subcarrier at the receiver, a short duration sample of the subcarrier known as the color burst is added to the composite video signal during the horizontal blanking interval following the horizontal sync pulse. The sound information is separated from the picture carrier by 4.5 megahertz. To further conserve channel bandwidth, the luminance signal and chrominance signal share a part of the channel bandwidth.
Thus, a low cost receiver is able to select the chrominance, sound and luminance signal portions by using appropriate frequency response networks or filters and thereafter perform individual processing thereon. Unfortunately, the frequency selection process used in most television receivers results in the loss of substantial amounts of information or image content. Perhaps the most notable loss occurs in the video or luminance information which is severely bandwidth limited as a result of the separation of chrominance information from the luminance information. In most receivers, this separation must be complete even if achieved at the expense of degrading either luminance or chrominance response, or both. While these losses have been recognized as less than desirable, the basic filtering processes used in most television receivers has made improvement difficult or impractical. In attempts to effectively separate chrominance and luminance, many receivers employ an analog glass delay line comb filter to separate luminance and chrominance information from the shared frequency spectrum. Since a glass delay line does not provide accurate delay, factory alignments are needed to accurately separate luminance and chrominance signals, all of which adds greatly to the cost of the receiver.
An alternative approach to television receiver design which promises to improve the recovery of information at the receiver is found in the use of digital signal processing rather than the more pervasive presently used analog signal processing. Several advantages are provided by digital signal processing. For example, the separation of chrominance and luminance information in a digital environment may be carried forward using comb filters which use accurate delay and therefore accurately separate the luminance and the chrominance signals. Effective comb filters are more easily realized in the digital environment. In addition, a variety of information processing techniques which require memory for temporary storage of information are facilitated in a digital environment due to the ease with which memory may be achieved. The result is more effective recovery of the luminance information within the chrominance frequency band.
While the use of sophisticated filtering techniques, such as comb filtering, improves the separation of the chrominance information from the luminance information, additional problems arise as practitioners strive to recover and use a maximum of the luminance information within the television signal. One such problem arises due to the presence of the above-mentioned chrominance burst signal within the horizontal blanking interval. This chrominance burst signal comprises eight to ten cycles of a sample of the chrominance subcarrier used to transmit the chrominance information which is placed shortly after the horizontal sync pulse in a signal portion known as the "back porch". While this location is well-suited to the recovery and separation of chrominance burst for use in the processing of the chrominance information, it is less than desirable in its possible effect upon luminance information processing.
For example, in most television receivers, the DC level for the recovered video signal is established during the horizontal blanking interval of the signal. The establishment of a correct DC level is critical in maintaining accurate luminance display as well as other related functions such as recovery of the scan synchronizing signals. Unfortunately, the back porch portion of the blanking signal is generally the reference level used to establish the correct DC signal component. In most systems, a clamping circuit responds to this back porch level and imposes the required DC component upon the signal.
The problem, however, arises from the fact that the chrominance burst signal occupies much of the back porch making DC clamping difficult and less reliable. This is particularly true in the above-mentioned digital systems which require precise timing to properly sample this small remaining portion of the back porch signal. Also, it is a basic characteristic of digital signal processing that reducing the number of samples used in any sampling operation may correspondingly reduce sample accuracy and reliability. The reliability of sampling a shorten signal portion may be improved through the use of accurate sample timing and faster sample rates. However, this increases system complexity and cost.
There remains, therefore, a need in the art for an improved television signal clamping system which overcomes the problems of chrominance burst signal presence.
Accordingly, it is a general object of the present invention to provide an improved video processor. It is a more particular object of the present invention to provide an improved video processor which effectively establishes the DC component for a composite video signal for television systems having a chrominance burst signal on the clamping reference.